The present invention relates to improvements in methods and apparatus for monitoring the diameters of rod-shaped products, especially rod-shaped products of the tobacco processing industry. Such products include continuous cigarette rods, sections of cigarette rods (i.e., plain cigarettes of unit length or multiple unit length), continuous filter rods, sections of filter rods (i.e., filter mouthpieces or plugs of unit length or multiple unit length), continuous cigarillo or cigar rods, sections of cigarillo or cigar rods, filter cigarettes, cigarillos or cigars and analogous commodities which constitute or form part of smokers' products. More particularly, the invention relates to improvements in methods and apparatus for monitoring the diameters of rod-shaped products which comprise or can embody tubular wrappers of porous material, such as cigarette paper, artificial cork and the like. Still more particularly, the invention relates to improvements in methods and apparatus which can be resorted to with advantage in connection with the monitoring of diameters of continuous rod-shaped products, such as cigarette rods or filter rods which advance axially in or beyond a cigarette or filter rod making machine on their way to a station where they are subdivided into rod-shaped sections of desired (unit or multiple unit) length. For the sake of simplicity, the following description will deal primarily with the making and monitoring of filter rods. However, the improved method and apparatus can be used with equal or similar advantage for the monitoring of diameters of cigarette rods as well as all other aforeenumerated rod-shaped products.
It is customary to monitor the diameter of a filter rod which is produced in a filter rod making machine wherein a web of cigarette paper or the like is draped around a continuous rod-like filler of fibrous or other filter material. The reason is that deviations of the diameter of a filter rod from a standard value can result in the making of long series of defective products. Thus, when a filter rod is subdivided into filter rod sections of desired length and such sections are assembled with plain cigarettes into filter cigarettes of unit length or multiple unit length, the diameter of the filter rod section must be the same as that of the plain cigarette. Otherwise, the customary uniting bands which are used to assemble filter rod sections with plain cigarettes will exhibit leaks and the resulting filter cigarettes will be ejected by the testing unit which inspects finished filter cigarettes in a filter tipping machine for the integrity of their wrappers. Moreover, differences between the diameters of filter rod sections and plain cigarettes in filter cigarettes detract from the appearance of filter cigarettes and interfere with proper introduction of such products into soft packs, hard packs or other types of receptacles. The unit which tests the wrappers of filter cigarettes for integrity must ascertain small and also minute deviations of permeability from optimum permeability, especially in view of the recently introduced ventilating zones which constitute intentionally formed perforations in order to allow for penetration of controlled quantities of fresh air into the column of tobacco smoke when the cigarette is lighted and the smoker draws smoke into his or her mouth.
Heretofore known diameter monitoring apparatus employ special types of nozzles wherein successive increments of the rod are acted upon by a gaseous testing fluid and which operate quite satisfactorily as long as the material of the wrapper or envelope is at least substantially impermeable to air. Reference may be had to commonly owned British Pat. No. 1.521,116 which discloses a nozzle for the monitoring of the diameter of a continuous filter rod and operates quite satisfactorily as long as the tubular envelope of the filter rod does not permit testing fluid to penetrate therethrough. However, such diameter monitoring apparatus become unreliable if the wrapper of the tested rod-shaped product is permeable to air which is invariably or practically invariably the case in modern filter rod making machines which assemble the filter rod from a rod-like filler of filter material and a tubular envelope which is made of porous or even highly porous wrapping material. At the very least, conventional diameter monitoring units are incapable of ascertaining relatively small deviations of the diameter of a continuous filter rod from an optimum value if the envelope of such filter rod consists of porous or highly porous material. Failure to immediately detect deviations of the diameter from an optimum value can result in the making of a long series of defective filter rod sections and of an equally long series of unsatisfactory filter cigarettes because a modern filter rod maker can turn out many thousands of filter rod sections per unit of time.